N2O or O2 is to be administrated by inhalation via a mouth/nose mask. N2O and O2 is delivered by the central hospital gas system. The mask contains a tight circle, where gas is only released from the mask by negative pressure from inspiration of the patient. This is to prevent leakage of gas to the environment. The mask is held by the patient and the patient controls the use of the mask (patient-controlled analgesia). The gas system has been approved by the local department of medical equipment. Measurements of N2O concentrations in the endoscopy room are frequently performed by an independent body, in accordance with Norwegian law.